1. Field of the Invention
The present invention generally relates to polymeric membranes that exhibit gas selectivity. Specifically, rigid polymeric membranes that exhibit an olefin/paraffin selectivity are described.
2. Description of the Related Art
The separation of one or more gases from a multicomponent mixture of gases is necessary in a large number of industries. Such separations currently are undertaken commercially by processes such as cryogenics, pressure swing adsorption, and membrane separations. In certain types of gas separations, membrane separations have been found to be economically more viable than other processes.
In a pressure-driven gas membrane separation process, one side of the gas separation membrane is contacted with a multicomponent gas mixture. Certain of the gases of the mixture permeate through the membrane faster than the other gases. Gas separation membranes thereby allow some gases to permeate through them while serving as a relative barrier to other gases. The relative gas permeation rate through the membrane is a property of the membrane material composition and its morphology.
It has been suggested in the prior art that the intrinsic permeability of a polymer membrane is a function of both gas diffusion through the membrane, controlled in part by the packing and molecular free volume of the material, and gas solubility within the material. Selectivity may be determined by the ratio of the permeabilities of two gases being separated by a material.
Transport of gases in polymers and molecular sieve materials occurs via a well known sorption-diffusion mechanism. The permeability coefficient (PA) of a particular gas is the flux (NA) normalized to the pressure difference across the membrane (ΔPA), and the membrane thickness (l).
                              P          A                =                              N            A                    ⁢                      l                          Δ              ⁢                                                          ⁢                              p                A                                                                        (        1        )            
The permeability coefficient of a particular penetrant gas is also equal to the product of the diffusion coefficient (DA) and the solubility coefficient (SA).PA=DASA  (2)
The permselectivity (αA/B) of a membrane material (also ideal selectivity) is the ratio of the permeability coefficients of a penetrant pair for the case where the downstream pressure is negligible relative to the upstream feed pressure. Substituting equation (2), the ideal permselectivity is also a product of the diffusivity selectivity and solubility selectivity of the particular gas pair.
                              α                      A            /            B                          =                                            P              A                                      P              B                                =                                                    D                A                                            D                B                                      ·                                          S                A                                            S                B                                                                        (        3        )            
The variation of gas permeability with pressure in glassy polymers is often represented by the dual mode model. Petropulos (1970); Vieth, et al. (1976); Koros, et al. (1977). The model accounts for the differences in gas transport properties in an idealized Henry's law and Langmuir domains of a glassy polymer,
                    P        =                                            k              D                        ⁢                          D              D                                +                                                    C                H                ′                            ⁢                              D                H                            ⁢              b                                      1              +              bp                                                          (        4        )            where kD is the Henry's law constant, C′H is the Langmuir capacity constant, p is pressure, and b is the Langmuir affinity constant. This model can be further extended to mixed gas permeability:
                              P          A                =                                            k              DA                        ⁢                          D              DA                                +                                                    C                HA                ′                            ⁢                              b                A                            ⁢                              D                HA                                                    1              +                                                b                  A                                ⁢                                  p                  A                                            +                                                b                  B                                ⁢                                  p                  B                                                                                        (        5        )            where pA and pB are the partial pressures of gasses A and B respectively. This model is valid for a binary gas mixture of components A and B, and it only accounts for competitive sorption.
The temperature dependence of permeability for a given set of feed partial pressures is typically represented by an Arrhenius relationship:
                    P        =                              P            o                    ⁢                      exp            ⁡                          [                                                -                                      E                    p                                                  RT                            ]                                                          (        6        )            where Po is a pre-exponential factor, Ep is the apparent activation energy for permeation, T is the temperature of permeation in Kelvin, and R is the universal gas constant. The permeability can further be broken up into temperature dependent diffusion and sorption coefficients from equation (2). The temperature dependence of the penetrant diffusion coefficient can also be represented by an Arrhenius relationship:
                    D        =                              D            o                    ⁢                      exp            ⁡                          [                                                -                                      E                    d                                                  RT                            ]                                                          (        7        )            
Again Do is a pre-exponential factor, and Ed is the activation energy for diffusion. The activation energy for diffusion represents the energy required for a penetrant to diffuse or “jump” from one equilibrium site within the matrix to another equilibrium site. The activation energy is related to the size of the penetrant, the rigidity of the polymer chain, as well as polymeric chain packing. The temperature dependence of sorption in polymers may be described using a thermodynamic van't Hoff expression:
                    S        =                              S            o                    ⁢                      exp            ⁡                          [                                                -                                      H                    s                                                  RT                            ]                                                          (        8        )            where So is a pre-exponential factor, and Hs is the apparent heat of sorption as it combines the temperature dependence of sorption in both the Henry's law and Langmuir regions.
From transition state theory the pre-exponential for diffusion can be represented by
                              D          o                =                              ⅇλ            2                    ⁢                      kT            h                    ⁢                      exp            ⁡                          [                                                S                  d                                R                            ]                                                          (        9        )            
Here, Sd is the activation entropy, λ is the diffusive jump length, k is Boltzmann's constant, and h is Planck's constant. Substituting (9) into (3) (neglecting small differences in the jump length of similarly sized penetrants) results in the diffusive selectivity as the product of energetic and entropic terms:
                                          D            A                                D            B                          =                              exp            ⁡                          [                                                                    -                    Δ                                    ⁢                                                                          ⁢                                      E                                          d                      ,                      A                      ,                      B                                                                      RT                            ]                                ⁢                      exp            ⁡                          [                                                ΔS                                      d                    ,                    A                    ,                    B                                                  R                            ]                                                          (        10        )            
The diffusivity selectivity is determined by the ability of the polymer to discriminate between the penetrants on the basis of their sizes and shapes, and is governed primarily by intrasegmental motions and intersegmental packing. The diffusive selectivity will be based on both the difference in activation energy for both penetrants, ΔEd, as well as the difference in activation entropy for both penetrants, ΔSd.
Much of the work in the field has been directed to developing membranes that optimize the separation factor and total flux of a given system. It is disclosed in U.S. Pat. No. 4,717,394 to Hayes that aromatic polyimides containing the residue of alkylated aromatic diamines are useful in separating a variety of gases. Moreover, it has been reported in the literature that other polyimides, polycarbonates, polyurethanes, polysulfones and polyphenyleneoxides are useful for like purposes. U.S. Pat. No. 5,599,380 to Koros, herein incorporated by reference, discloses a polymeric membrane with a high entropic effect. U.S. Pat. No. 5,262,056 to Koros et al., herein incorporated by reference, discloses polyamide and polypyrrolone membranes for fluid separation.
U.S. Pat. No. 5,074,891 to Kohn et al. discloses certain polyimides with the residuum of a diaryl fluorine-containing diamine moiety as useful in separation processes involving, for example, H2, N2, CH4, CO, CO2, He and O2. By utilizing a more rigid repeat unit than a polyimide, however, even greater permeability and permselectivity are realized. One example of such a rigid repeat unit is a polypyrrolone.
Polypyrrolones as membrane materials were proposed and studied originally for the reverse osmosis purification of water by Scott et al. (1970). The syntheses, permeabilities, solubilities and diffusivities of polypyrrolones and polyimides have been described in (Walker and Koros (1991); Koros and Walker (1991); Kim et al. (1988a, b); Kim (1988c); Coleman (1992)). Membranes that are composed of the polyamide and polypyrrolone forms of hexafluoroisopropylidene-bisphthalic anhydride are disclosed in U.S. Pat. No. 5,262,056 which is incorporated herein by reference.
In the petrochemical industry, one of the most important processes is the separation of olefin and paraffin gases. Olefin gases, particularly ethylene and propylene, are important chemical feed stocks. Various petrochemical streams contain olefins and other saturated hydrocarbons. These streams typically originate from a catalytic cracking unit. Currently, the separation of olefin and paraffin components is done using low temperature distillation. Distillation columns are normally around 300 feet tall and contain over 200 trays. This is extremely expensive and energy intensive due to the similar volatilities of the components.
It is estimated that 1.2×1014 BTU per year are used for olefin/paraffin separations. This large capital expense and exorbitant energy cost have created incentive for extensive research in this area of separations. Membrane separations have been considered as an attractive alternative. Some examples of membranes that exhibit attractive selectivity under mild conditions have been reported. (Tanaka et al. (1996); Staudt-Bickel and Koros (2000); Ilinitch et al. (1993); Lee et al. (1992); Ito et al. (1989)). In practice, high propylene/propane temperatures and pressures are preferred for economical processing. Thus, a polymer membrane that showed enhanced propylene/propane selectivity under increasingly demanding processing conditions would be of particular value. Recent gas transport studies aimed at improving current membrane performance have examined glassy polymers focusing mainly on polyimides. Tanaka et al. (1996) have reported one of the highest performance polyimides to date. This data along with other literature data has been used to construct a preliminary propane/propylene “upper bound” trade off curve between gas permeability and selectivity, as shown in FIG. 1 (Tanaka et al. (1996); Staudt-Bickel and Koros (2000); Ilinitch et al. (1993); Lee et al. (1992); Ito et al. (1989); Steel (2000)). The conditions chosen for the upper bound curve are 2–4 atm feed pressure and 35–55° C. The propane/propylene trade off curve is less defined at this point in comparison to O2/N2 and CO2/CH4 “upper bound” curves defined previously (Robeson (1991)).
According to Freeman's analysis, the “upper bound” for conventional polymers used for gas separations can be shown to follow equation 11:αA/B=(βA/B)/(PAλA/B)  (11)The parameter, λA/B can be shown to be proportional to the square of the size difference of the two gas molecules, (dA/dB)2. Consequently, this parameter is difficult to manipulate through materials engineering. Therefore, according to the theory, in order to “move” the upper bound limit, the strategy must be to increase the β parameter, which can be shown to be proportional to the value, SA (SA/SB)λ, as well as a parameter f, which relates to the interchain spacing. Previous work has attempted to increase the diffusivity selectivity through an increase in the chain rigidity by using polypyrrolone materials. However, another approach to “elevating” the upper bound is to improve the solubility of the “fast gas” (i.e., C3H6 in this case), thereby increasing the solubility selectivity, and increasing β. The solubility of an olefin in a polymeric material is a parameter that can be engineered through the use of π-bonding interactions.
Previous researchers have examined the viability of fixed site facilitated membranes for the separation of olefins from paraffins. Typically, Group I-B metals, such as silver, are dissolved in polymer membranes in a salt form. Examples of silver salts conventionally used include AgBF4 and AgNO3. Once dissolved in the polymer, the salt dissociates, and the silver cation is able to form a complex with an olefin due to the interaction of the π-orbital of the olefin with the σ and π-orbital of the metal.
These fixed site facilitated membranes still have a major practical problem, however, due to the poor chemical stability of the metal-olefin complex. This metal-olefin complex is easily poisoned by small amounts of hydrogen gas, carbon monoxide, acetylene, or hydrogen sulfide in the feed stream. Silver ions also have the potential to react with acetylene to form an explosive silver acetylide salt.
A search has been ongoing to find a material which can form a π-bond complex with olefins, while still maintaining stability in the presence of the aforementioned impurity gases. An additional constraint is that the material should be able to dissolve in state-of-the-art polyimide membranes. The overall strategy is to maintain the high diffusivity selectivity (DA/DB) already available with polyimides, and enhance this diffusivity selectivity by a factor of the improved solubility selectivity (SA/SB). For a conventional polyimide with a C3H6/C3H8 selectivity of 15, a small increase in the solubility selectivity from 1.0 to 2.0 would double the overall C3H6/C3H8 selectivity to a value of 30, as well as doubling the C3H6 flux.